miracle, myth…or marketing Wood chip mulch: Landscape boon or bane?

andscape mulches are increasingly An exhaustive review of the science recognized as pivotal components of behind landscape mulches is beyond environmentallyL sustainable gardens and the scope of this column (though I green spaces. Select the right mulch and have just completed such a review for Linda Chalker-Scott, Ph.D. you reap the benefits of healthier soils and upcoming publication in a scientific MasterGardener WSU editor plants. Choose the wrong mulch and the journal). Instead, I’m going to address Extension Urban Horticulturist only plants that thrive are the weeds. the documented benefits and drawbacks and Associate Professor, Before selecting a landscape mulch behind the use of arborist wood chips as a Puyallup Research and Extension Center, material, it’s important to reflect landscape mulch. Washington State University on the purpose of the landscape in Puyallup, Washington question. For instance, production Perfect choice agriculture generally requires short In areas where trees are a dominant term, intensive management of a crop, feature of the landscape, arborist wood while the philosophy behind landscape chips represent one of the best mulch horticulture is the long term, sustainable choices for trees and shrubs. A 1990 management of a system. Therefore, study evaluated the landscape mulch those mulches that work best for crop potential of 15 organic materials, production (including vegetable gardens) including grass clippings, leaves, are often not the best choices for woody composts, yard wastes, bark, and wood ornamental landscapes, and vice versa. chips. Wood chips were one of the best performers in terms of moisture Direct benefits retention, temperature moderation, weed The potential,direct benefits of any control, and sustainability. In many landscape mulch material can be grouped urban areas, arborist wood chips are Interested in into four general categories: available for free, representing one of the field trial research? Soil benefits most economically practical choices. Is your local Master Gardener program • improve structure Unlike the uniform nature of interested in conducting field trials • enhance gas transfer sawdust and bark mulches, wood chips on gardening products? Occasionally, • enhance water infiltration and include bark, wood, and often leaves. companies will contact Dr. Chalker-Scott with requests for product testing. retention The chemical and physical diversity Interested groups will receive basic training • prevent erosion and compaction of these materials resists the tendency in designing and conducting experimental • moderate temperature towards compaction seen in sawdust and field trials; local programs will need to Plant benefits bark. Additionally, the materials vary provide all materials and field space. • If interested, please email Dr. Chalker-Scott provide nutrients in their size and decomposition rate, at [email protected].. System benefits creating a more diverse environment that • suppress pathogens and pests is subsequently colonized by a diverse • enhance beneficial organisms soil biota. A biologically diverse soil MORE INFORMATION • increase biodiversity biota is more resistant to environmental • Dr. Chalker-Scott’s extensive neutralize pollutants disturbance and will in turn support a reference information is located Human benefits diverse and healthy plant population. at the following Web site: • economic Wood chips are considered to be www.puyallup.wsu.edu/~Linda%20Chalker- • aesthetic slow decomposers, as their tissues are Scott/Wood%20chip%20references.html • ease of application rich in lignin, suberin, tannins, and

SUMMER 2007 www.MasterGardenerOnline.com • MASTERGARDENER • PAGE 21 other decomposition-resistant, natural them out of the landfill—a benefit with (e.g. Juglans nigra, black walnut). Cedars compounds. Thus, wood chips supply both economic and environmental (Thuja spp.) have not been found to have nutrients slowly to the system; at the same attributes. this ability. time they absorb significant amounts Concern: Mulches made from of water that is slowly released to the Drawbacks of wood chips— chipping diseased trees can infect healthy soil. It is not surprising that wood chips mulch ado about nothing trees. have been cited as superior mulches for There are a number of concerns Evidence: Most studies indicate enhanced plant productivity. Wood chips surrounding the use of arborist wood that diseased mulch cannot transmit have been especially effective in helping chips as a landscape mulch. I have pathogens to the roots of healthy trees. establish trees and native plants in urban constructed a quick summary here. Under no circumstances should wood and disturbed environments. Relevant references can be found on the mulch be used as backfill. Not only is Arborist wood chips provide Web site listed in “More Information.” this a poor installation practice, but a incredible weed control in ornamental Overall, the commonly expressed potential mechanism for disease transfer landscapes. The mechanism(s) by which concerns about woody mulches are not as well. Fungal communities found wood chips prevent weed growth are borne out in research trials. in wood chip mulches are generally not fully understood, but probably Concern: Woody mulches will decomposers, not pathogens. Under include light reduction (preventing acidify soils. healthy soil conditions, beneficial germination of some seeds and reducing Evidence: None. In field situations and harmless fungi can out-compete photosynthetic ability of buried leaves), it is difficult to significantly alter soil pH pathogens for space on plant roots. allelopathy (inhibiting seed germination), without addition of chemicals. Transient Furthermore, healthy plants are not and reduced nitrogen levels at the changes in pH may be found in the susceptible to opportunistic pathogens soil-mulch interface (reducing seedling decomposing mulch layer itself, but these such as Armillaria and Phytophthora, survival). have little effect on underlying soils. which are often ubiquitous but inactive in While there are imported wood Concern: Woody mulches, such as well-managed soils. mulches available for purchase at nurseries cedar, leach allelopathic chemicals that Concern: Wood chips could be a fire and home improvement centers, they are kill other plants. hazard, particularly when they are used on not as cost-effective as locally produced Evidence: Many plant materials landscapes around structures. wood chips, which are often free. In a contain allelopathic chemicals, which Evidence: Coarse textured organic society where using locally produced can prevent seeds from germinating or mulches, like wood chips, are the least materials is increasingly popular as a kill young seedlings. Most compounds flammable of the organic mulches. Fine measure of sustainability, arborist wood have no effect upon established plants. textured mulches are more likely to chips are a natural choice. Finally, the Only a few woody materials have been combust, and rubber mulch is the most reuse of plant materials as mulches keeps found to contain allelopathic chemicals hazardous of all tested landscape mulches.

A bargain Jim Black for gardeners

Arborist wood chips are available locally in many communities. Most sources are free or inexpensive for wood chips made from recycled pallets and other discarded wood products. That this mulch is both cheap and scientifically proven to be superior makes it an ideal Master Gardener-recommended product. At left, Yakima County recyling and compost education program coordinator Mikal Heintz is shown with her wood chip operation located at the county landfill. Former head of the Yakima Area Arboretum, Mikal works with the community to promote composting and wood chip mulches.

PAGE 22 • MASTERGARDENER • www.MasterGardenerOnline.com SUMMER 2007 Concern: Wood chip mulches will Arborist wood mulch, when used with tie up nitrogen and cause deficiencies in contrasting landscape textures, such plants. as the gravel, rock, and grass shown at : Actually, many studies right, produces a low-maintenance but Evidence beautiful garden setting. have demonstrated that woody mulch materials increase nutrient levels in soils and/or associated plant foliage. My hypothesis is that a zone is most effective in suppressing weeds of nitrogen deficiency exists at the when weeds are not yet present on site. mulch/soil interface, inhibiting weed seed Therefore, bare soil should be mulched as germination while having no influence soon as practical, especially in the spring upon established plant roots below and fall when weed seed germination is at the soil surface. For this reason, it is its peak. If this is not possible, the most inadvisable to use high C:N mulches in effective, non-chemical way to remove annual beds or vegetable gardens where weeds prior to mulching is to mow the plants of interest do not have deep, them as close to the ground as possible, extensive root systems. followed immediately by mulching. Concern: Woody mulches will attract Prune or mow perennial weeds at termites, carpenter ants, and other pests. the root crown in early spring when root Evidence: Many wood-based mulches resources are lowest (generally just as are not attractive to pest insects but are leaf growth begins). Extensive pulling of actually insect repellent. For instance, perennial weeds from unprotected soil cedar (Thuja) species produce thujone, is not recommended, as this disturbance which repels clothes moths, cockroaches, will increase erosion, especially in sandy termites, carpet beetles, Argentine ants, soils or in sloped areas. It is better to keep and odorous house ants. In general, unprotected soil undisturbed. However, termites prefer higher nutrient woody you can pull resprouting perennial materials, such as cardboard, rather than weeds covered in mulch; the mulch layer wood chips. prevents erosion and facilitates pulling. Remove all noxious weed materials Application from site to prevent rerooting or seeding. Let wood chips age before using Self explanatory! them if there are concerns about disease. Install chips to the desired depth. A Personally, I have never done this; I successful wood chip mulch must be deep happen to love the smell of fresh wood enough to suppress weeds and promote chips and enjoy spreading them out over healthy soils and plants: research has the landscape. Additionally, some of the demonstrated that weed control is directly nutrient value (particularly nitrogen if linked to mulch depth, as is enhanced the chips contain leaves or needles) will plant performance. A review of the borers are more likely to invade a plant be lost in the composting process. Using research on coarse organic mulches and whose bark is wet due to excessive fresh chips ensures that some of the foliar weed control reveals that shallow mulch mulching. Rather than creating mulch nitrogen will feed the landscape rather layers will promote weed growth and/or volcanoes, instead taper the mulch down than the compost pile. require additional weed control measures. to nearly nothing as you approach the Before installing wood chips, create a I recommend 4-6 inches for ornamental trunk. This donut-shaped application will thin underlying layer of a more nutrient- sites and 8-12 inches for restoration sites protect the soil environment as well as the rich mulch (like compost) if there are and/or perennial weed problems. above-ground plant tissues. concerns about nutrient deficiencies. Keep mulch away from trunks of Replace mulch as needed to maintain This “mulch sandwich” approach is trees and shrubs. Piling mulch against desired depth; replacement rate will a logical one that mimics what you the trunks of shrubs and trees creates a depend on decomposition rate. Once would see in the mulch layer of a forest dark, moist, low oxygen environment mulch is applied, little management needs ecosystem. It’s not required, though, and to which above-ground tissues are not to be done other than reapplication to over time a wood chip mulch will develop adapted. Fungal diseases require a moist maintain minimum depth. High traffic this same structure as the lower layers environment to grow and reproduce; areas are most likely to need replacement. break down. piling mulch on the trunk provides n Begin mulch application before exactly the right conditions for fungi to annual weeds are established. Mulch enter the plant. Likewise, opportunistic

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